Update README badges (#1240)
Update README badges (#1240)

Motivation:

grpc-swift is now a graduated SSWG project; the badge in our README
should reflect that.

Modifications:

- Update SSWG badge to graduated
- Update CI badge which was still pointing to Travis

Result:

README is more up-to-date
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift